Why are dfdl:discriminators so .... different?

[Costello, Roger L.]

Hi DFDL community,

Below is an xs:choice that is part of my DFDL schema for Windows executable 
files. Here is my understanding of implementing choices in DFDL:

An xs:choice consists of a series of sequences.
                Daffodil uses dfdl:descrimintor to decide which
               sequence to choose.

Is that correct?

Question: When I do this discriminator stuff, I blindly follow the pattern 
shown below, i.e., embed dfdl:discriminator within xs:appinfo. When I try to 
explain discriminators to people, I just know that they are going to ask: Why? 
Why are dfdl:discriminators embedded within xs:appinfo? Discriminators seem to 
be handled very different from everything else, why?

How do I respond to these questions?  /Roger


<xs:choice>
     <xs:sequence>
            <xs:annotation>
                <xs:appinfo source="http://www.ogf.org/dfdl/";>
                    <dfdl:discriminator 
test="{fn:lower-case(xs:string(Hidden_signature)) eq '0b01'}" />
                </xs:appinfo>
            </xs:annotation>

            element declarations for 32-bit executable

     </xs:sequence>
     <xs:sequence>
            <xs:annotation>
                <xs:appinfo source="http://www.ogf.org/dfdl/";>
                    <dfdl:discriminator 
test="{fn:lower-case(xs:string(Hidden_signature)) eq '0b02'}"/>
                </xs:appinfo>
            </xs:annotation>

            element declarations for 64-bit executable

     </xs:sequence>
     <xs:sequence>
            <xs:annotation>
                <xs:appinfo source="http://www.ogf.org/dfdl/";>
                    <dfdl:discriminator 
test="{fn:lower-case(xs:string(Hidden_signature)) eq '0701'}"/>
                </xs:appinfo>
            </xs:annotation>

            element declarations for ROM image

     </xs:sequence>

</xs:choice>